A stopper is fitted over a ring-shaped locking part of a support member and rotates in a manner in which a first protrusion thereof is able to move in a ring gap of the locking part. When a first leg part is rotated by a lever, a convex part (another stopper) rotates in accordance therewith so as to abut a second protrusion and cause the stopper to rotate. Furthermore, the distance between the off-center lower part and a second leg part will change, and it is through the change in pressing force against attachment holes that an anchored or non-anchored state of the lower part and the second leg part to the attachment holes is selected. In this case, the first protrusion abuts one of the two locking end parts of the locking part of the support member, restricting the range of rotation of the lever. Therefore, in the anchored state wherein the off-center leg part is anchored in the respective attachment hole, the lever for fastening the off-center leg part in the attachment hole is thus reliably retained in a prescribed position, and it is possible to prevent the lever from rotating in an unintended manner.
